1. Field of the Invention
The present invention relates to a heating furnace for elongating a glass rod such as optical fiber glass preform, and a method of elongating a glass rod.
2. Related Background Art
Generally known as a heating furnace for elongating a glass rod such as optical fiber glass preform is the one in which, as shown in FIG. 3, a glass rod 101 is heated and softened successively from the lower end side such as to be elongated. In this heating furnace, dummy rods 102 connected to both ends of the glass rod 101 are held with chucks 103A and 103B, respectively, and these chucks 103A and 103B are moved down such that the lower chuck 103B moves faster than the upper chuck 103A. The glass rod 101 is heated by a heater 107 from its lower end side, and is elongated by a tensile force which occurs due to the speed difference between the chucks 103A and 103B.
If oxygen intrudes into the heating furnace, carbon-based parts within the heating furnace such as the heater 107 and muffle tube 109 will deteriorate due to oxidization. For preventing such deterioration, the inside of the heating furnace is usually purged by an inert gas when elongating the glass rod 101.
In such a heating furnace, in order to prevent the outside air from intruding into the heating furnace and secure the elongating of the whole glass rod 101, the upper dummy rod 102 and a cylindrical portion 106A must have such a length that securely assures the longitudinal movement of the glass rod 101. Further, above the cylindrical portion 106A, it is necessary to secure a movable area for the chuck 103A holding the dummy rod 102, which would disadvantageously increase the size of the heating furnace as a whole. Hence,there has been employed a heating furnace which uses, instead of the cylindrical portion 106A, a bellows which expands and contracts depending on the movement of the glass rod as disclosed in Japanese Patent Application Laid-Open No. 6-256034, No. 2-51440, No. 61-1914536, No. 53-24429, and the like. FIG. 4 shows an example of a heating furnace having such a bellows. In the heating furnace in FIG. 4, constituent parts identical or equivalent to those in the heating furnace shown in FIG. 3 are referred to with numerals or letters identical thereto.